Bag filter comprising polyphenylene sulfide and acrylic fiber

ABSTRACT

This invention relates to a bag filter having a tubular section, one closed end and one open end, the tubular section having a nonwoven felt comprising an intimate fiber blend of 50 to 80 parts by weight polyphenylene sulfide fiber and 20 to 50 parts by weight acrylic fiber, based on the total weight of polyphenylene sulfide and acrylic fibers in the felt, wherein the acrylic fiber has an equal or lower denier per filament than the polyphenylene sulfide fiber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to bag filters having enhanced filtrationperformance. Such bag filters are particularly useful in filtering hotgases, for example gases at temperatures of 150° C. or even higher. Inone embodiment, the bag filters are particularly useful in power plantsto meet emission standards.

2. Description of Related Art

Polyphenylene sulfide bag filters for hot gas filtration such asdisclosed in European Patent Application 386 975 of Chapman et al. areknown and are used to protect the environment from particulate matterfrom asphalt plants, coal plants, and other industrial concerns. Due tothe high potential environmental impact from such plants, anyimprovement that has the potential to improve the capture of particulatematerial per unit weight of bag filter is desired.

BRIEF SUMMARY OF THE INVENTION

This invention relates to a bag filter having a tubular section, oneclosed end and one open end, the tubular section having a nonwoven feltcomprising an intimate fiber blend of 50 to 80 parts by weightpolyphenylene sulfide fiber and 20 to 50 parts by weight acrylic fiber,based on the total weight of polyphenylene sulfide and acrylic fibers inthe felt, wherein the acrylic fiber has an equal or lower denier perfilament than the polyphenylene sulfide fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE illustrates one embodiment of the bag filter of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a bag filter made from an intimate fiber blendof 50 to 80 parts by weight polyphenylene sulfide fiber and 20 to 50parts by weight acrylic fiber wherein the acrylic fiber has an equal orlower denier per filament than the polyphenylene sulfide fiber.Surprisingly the inventors have found that certain acrylic fibers, whichare generally thought to not be suitable for use in hot gasenvironments, can improve filtration performance of bag filters. It isbelieved acrylic fibers work with the polyphenylene sulfide fibers in abag filter to retain more particulates without adversely affecting thefilter bag mechanical properties.

The FIGURE illustrates one embodiment of the filter bag of thisinvention. Filter bag 1 has a closed end 2, an open end 3, and a tubularsection 4. In the embodiment represented, the filter bag also has aspring steel metal snap ring 5 attached to the open end of the bag. Thetubular section 4 of this bag is comprised of a filtration felt that isoverlapped, forming a seam 6 sewn with triple stitching 7. The closedend of the bag in this embodiment is also comprised of a filtration feltthat is stitched at 8 to the end of the felt used for the tubularsection. While the FIGURE represents a preferred embodiment, otherpotential constructions, orientations, and features of bag filters maybe used, such as those disclosed in U.S. Pat. No. 3,524,304 toWittemeier et al.; U.S. Pat. No. 4,056,374 to Hixenbaugh; U.S. Pat. No.4,310,336 to Peterson; U.S. Pat. No. 4,481,022 to Reier; U.S. Pat. No.4,490,253 to Tafara; and/or U.S. Pat. No. 4,585,833 to Tafara.

In some embodiments the closed end 2 of the filter bag, as shown in theFIGURE, is a disk of filter material sewn to the tubular section. Insome other embodiments the closed end can be made of some othermaterial, for example in some situations a metallic closed end might beneeded. In other embodiments the closed end can be ultrasonically,adhesively, or heat seamed or sealed in some other manner than sewing.In another embodiment the felt used in the tubular section of the bagcan be gathered together or folded, and then sealed, to form the closedend.

In some embodiments the open end 3 of the bag may be provided withhardware to attach the bag to the cell plate. In some other embodimentsthe open end of the bag may be sized such that a snug fit isaccomplished by sliding the bag over a specially designed cell plate.

In some embodiments of this invention the filtration material used inthe tubular section 4 and optionally in the closed end 2 is a nonwovenfabric or felt. The nonwoven fabric or felt can be made by conventionalnonwoven sheet forming processes, including processes for makingair-laid nonwovens, wet-laid nonwovens, or nonwovens made from cardingequipment; and such formed sheets can be consolidated into fabrics viaspunlacing, hydrolacing, needlepunching, or other processes which cangenerate a nonwoven sheet. The spunlaced processes disclosed in U.S.Pat. Nos. 3,508,308 and 3,797,074; and the needlepunching processesdisclosed in U.S. Pat. Nos. 2,910,763 and 3,684,284 are examples ofconventional methods well known in the art that are useful in themanufacture of the nonwoven fabrics and felt.

In some preferred embodiments the nonwoven felt is a needledpunchedfelt; in some other preferred embodiments the nonwoven felt is aspunlaced felt. The basis weight of the felt is typically about 8 to 16ounces per square yard, and in a preferred embodiment is 12 to 14 ouncesper square yard.

In some embodiments the tubular section 4 optionally in the closed end 2of the filter bag of this invention is a single layer of filtrationmaterial. In some other embodiments the tubular section is made of afiltration material supported by a scrim or reinforcing cloth thatprovides stability during pulsing of the bag. In some preferredembodiments the nonwoven felt includes a supporting woven scrim that ismade with fibers that are compatible with the staple fibers in the felt.This type of felt can be made using standard carding and cross lappingequipment to convert staple fibers into crosslapped batts having basisweights of about 4 to 8 ounces per square yard (135 to 270 grams persquare meter) preferably about 6 ounces per square yard (200 grams persquare meter). If desired, the batts can then be tacked or lightlyconsolidated, for example, on a standard needle punch machine. Two ormore of these batts can then be positioned on either side of a wovenscrim having a basis weight of about 1 to 4 ounces per square yard (34to 135 grams per square meter) preferably about 2 ounces per square yard(70 grams per square meter), and the three layers are needled punchedseveral times on both sides to produce filtration felts. In somepreferred embodiments the woven scrim comprises polyphenylene sulfidefibers, meta-aramid fibers, or mixtures thereof.

In a preferred embodiment shown in the FIGURE the filtration material isoverlapped to form a cylinder of filter material having a seam 6 that isthen stitched with a high temperature thread, such as a thread having 3to 6 strand plies of aramid fiber, fluoropolymer fiber, glass fiber, orcombinations or blends thereof. In other embodiments, the overlappedseam can be sealed by ultrasonics, adhesives, heat, or some combinationof all these seaming methods.

One feature of the bag filter of this invention is that it includes anonwoven felt comprising an intimate fiber blend of 50 to 80 parts byweight polyphenylene sulfide fiber and 20 to 50 parts by weight acrylicfiber, based on the total weight of polyphenylene sulfide and acrylicfibers in the felt. In a preferred embodiment the intimate fiber blendcomprises 65 to 75 parts by weight polyphenylene sulfide fiber. Inanother preferred embodiment, the intimate fiber comprises 25 to 35parts by weight acrylic fiber. The fibers are disposed in the felt as anintimate blend, meaning that the polyphenylene sulfide and acrylicfibers are uniformly mixed and distributed in the felt. This forms auniform mixture in the felt so as to avoid hot spots or localized areascaused by having a high concentration of the acrylic fiber in any oneportion of the felt.

The intimate fiber blend can be formed by conventional methods. Forexample, in one embodiment, clumps of crimped staple fibers obtainedfrom bales of fiber can be opened by a device such as a picker and thenblended by any available method, such as air conveying, to form a moreuniform mixture. In an alternative embodiment, the fibers can be blendedto form a uniform mixture prior to fiber opening in the picker. In stillanother possible embodiment the fibers may be cutter blended, that is,tows of the various fiber types can be combined and then cut intostaple. The blend of fibers can then be converted into a nonwoven fabricor felt using conventional methods as mentioned previously. Generallythis involves forming a fibrous web by use of a device such as a card,although other methods, such as air-laying or wet-laying of the fibersmay be used. If desired the fibrous web can then be sent via conveyor toa device such as a crosslapper to create a crosslapped structure bylayering individual webs on top of one another in a zigzag structure.

Another feature of the bag filter of this invention is that the acrylicfiber has an equal or lower denier per filament, or linear density perfilament, than the polyphenylene sulfide fiber to enhance particleretention in the felt and improve bag filter performance. In someembodiments, the polyphenylene sulfide fiber has a linear density offrom about 1.5 to 3 denier per filament (1.7 to 3.3 decitex perfilament) and the acrylic fiber has a linear density of from about 0.5to 3 denier per filament (0.6 to 3.3 decitex per filament). In onepreferred embodiment, the linear density of the polyphenylene sulfidefiber is greater than 1.8 denier per filament (2.0 decitex per filament)up to about 2.2 denier per filament (2.4 decitex per filament); and theacrylic fiber has a linear density of from about 0.9 denier per filament(1.0 decitex per filament) up to 1.8 denier per filament (2.0 decitexper filament). The fibers used in this invention preferably have a cutlength of about 1.5 to 3 inches (38 to 76 mm) and are crimped, having acrimp frequency of about 4 to 10 crimps per inch (1.5 to 4 crimps percentimeter).

Acrylic polymer has a lower specific gravity (about 1.16) thanpolyphenylene sulfide polymer (about 1.34 to 1.38), meaning that forfibers having equal deniers, the filament diameter of an acrylic fiberwill be larger than the filament diameter of a polyphenylene sulfidefiber and per unit weight the surface area of the acrylic fiber islarger than the polyphenylene sulfide fiber. Therefore, when a bagfilter made from a felt of acrylic fiber and the polyphenylene sulfidefiber having the same filament denier is compared with a bag filter froma felt of solely polyphenylene sulfide fiber of the same filamentdenier, the bags will have identical unit weight; however, theacrylic/polyphenylene sulfide bag filter will have more fiber surfacearea which can capture particles. When the acrylic fiber has a lowerfilament denier than the polyphenylene sulfide fiber, more acrylicfibers can be incorporated into the bag filter per unit weight, againresulting in improved filtration performance due to improved surfacecoverage by the increased number of filaments. On the other hand, if thefilament denier of the acrylic fiber is appreciably larger than thefilament denier of the polyphenylene sulfide fiber, the acrylic fiberhas higher surface area, but on a per unit weight basis, fewer acrylicfibers must be used, resulting in a bag filter that is believed to havepoorer filtration efficiency.

The acrylic fiber useful in this invention includes acrylonitrile unitswhich are at least 85 weight % of the total acrylic fiber. Anacrylonitrile unit is —(CH₂—CHCN)—. The acrylic fiber can be made fromacrylic polymers made up of 85% by weight or more of acrylonitrile with15% by weight or less of an ethylenic monomer copolymerizable withacrylonitrile and mixtures of two or more of these acrylic polymers.Examples of the ethylenic monomer copolymerizable with acylonitrileinclude acrylic acid, methacrylic acid and esters thereof (methylacrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate,etc.), vinyl acetate, vinyl chloride, vinylidene chloride, acrylamide,methacylamide, methacrylonitrile, allylsulfonic acid, methanesulfonicacid and styrenesulfonic acid. One illustrative method of making acrylicpolymers and fibers is disclosed in U.S. Pat. No. 3,047,455. Acrylicfibers have been commercially manufactured by a number of companiesincluding Solutia, Inc. and Bayer Inc.; one particularly preferredacrylic fiber is commercially available from Sterling Fibers, Inc. ofPace, Fla.

The polyphenylene sulfide fibers useful in this invention have good heatresistance, chemical resistance and hydrolysis resistance, and at least90% of the constituent units of the fibers are of a polymer havingphenylene sulfide structural units of —(C₆H₄—S)—. Polyphenylene sulfidefiber is sold under trade names such as Toray PPS®, Forton®, Ryton® andProcon® by companies such as Toray Industries Inc., Kureha ChemicalIndustry Co., and Toyobo Co.

Test Methods

Filtration efficiency for filter bags can be measured using ASTM D 6830;mullen burst can be measured using ASTM D 461; and breaking strength andelongation can be measured using ASTM D 5035.

EXAMPLE 1

An intimate fiber blend containing 67 parts by weight of a 2 denier perfilament (2.2 decitex per filament) polyphenylene sulfide fiber having a2 inch (50 mm) cut length and 33 parts by weight of a 0.9 denier perfilament (1.0 decitex per filament) acrylic fiber having a 2 inch (50mm) cut length is made by combining and mixing the staple fibers frombales. Using standard carding and cross lapping equipment, these fibersare converted into crosslapped batts having a basis weight ofapproximately 6 ounces per square yard (200 grams per square meter) thatare then tacked or lightly consolidated on a standard needle punchmachine. Two of these batts are positioned on either side of a wovenscrim having a basis weight of approximately 2 ounces per square yard(70 grams per square meter) and consisting entirely of polyphenylenesulfide spun yarns, and the three layers are needled punched severaltimes on both sides to produce a nominal 14 ounce per square yard (475grams per square meter) filtration felt. The felts can then befabricated into filter bags that are approximately 120 inches (305 cm)long having a 7 inch (18 cm) double flat width and a 5 inch (13 cm) snapring top. The bag can also have triple-stitching at the seams.

1. A bag filter having a tubular section, one closed end and one openend, the tubular section having a nonwoven felt comprising an intimatefiber blend of a) 50 to 80 parts by weight polyphenylene sulfide fiberand b) 20 to 50 part by weight acrylic fiber, based on the total weightof polyphenylene sulfide and acrylic fibers in the felt, wherein theacrylic fiber has an equal or lower denier per filament than thepolyphenylene sulfide fiber; and wherein the bag filter is capable offiltering a hot gas at a temperature of at least 150 degrees centigrade.2. The bag filter of claim 1 comprising 65 to 75 parts by weightpolyphenylene sulfide fiber.
 3. The bag filter of claim 1 comprising 25to 35 parts by weight acrylic fiber.
 4. The bag filter of claim 1wherein the nonwoven felt is a needlepunched felt.
 5. The bag filter ofclaim 1 wherein the nonwoven felt is a spunlaced felt.
 6. The bag filterof claim 1 wherein the denier per filament of the polyphenylene sulfidefiber is from 1.5 to 3.0.
 7. The bag filter of claim 1 wherein thedenier per filament of the acrylic fiber is 0.5 to
 3. 8. The bag filterof claim 1 wherein the closed end also comprises the intimate fiberblend.
 9. The bag filter of claim 1 wherein the basis weight of the feltis from 8 to 16 ounces per square yard.
 10. The bag filter of claim 9wherein the basis weight of the felt is from 12 to 14 ounces per squareyard.
 11. A process of filtering a hot gas comprising passing a gasheated to a temperature of at least 150 degrees C. through a bag filter,having a tubular section, one closed end and one open end, the tubularsection having a nonwoven felt comprising an intimate fiber blend of a)50 to 80 parts by weight polyphenylene sulfide fiber and b) 20 to 50part by weight acrylic fiber, based on the total weight of polyphenylenesulfide and acrylic fibers in the felt, wherein the acrylic fiber has anequal or lower denier per filament than the polyphenylene sulfide fiber.